Pulmonary arterial hypertension (PAH), one of the five types of pulmonary hypertension (PH), is a life-threatening disease characterized by pulmonary vascular remodeling that leads to increased pulmonary vascular resistance and pulmonary arterial pressure, most often resulting in right-side heart failure. It is a progressive condition characterized by elevated pulmonary arterial pressures leading to right ventricular (RV) failure. It is defined at cardiac catheterization as a mean pulmonary artery pressure of 25 mm Hg or more. The most common symptom associated is breathlessness, with impaired exercise capacity as a hallmark of the disease.
Pulmonary hypertension can be classified as either primary or secondary. When the arterial hypertension is not accompanied, or caused by another underlying heart or lung disease or condition, it is called primary pulmonary arterial hypertension. When the arterial hypertension is triggered by another disease state, it is designated secondary arterial pulmonary hypertension. Exemplary conditions which can cause secondary pulmonary hypertension include congenital heart defects, ventricular or atrial septal defects/holes, which are in some cases called Eisenmenger complex, as well as valve conditions such as stenosis.
PAH is associated with significant morbidity and mortality. It is caused by complex pathways that culminate in structural and functional alterations of the pulmonary circulation and increases in pulmonary vascular resistance and pressure. Many mechanisms can lead to elevation of pulmonary pressures. In PAH, progressive narrowing of the pulmonary arterial bed results from an imbalance of vasoactive mediators, including prostacyclin, nitric oxide, and endothelin-1. This leads to an increased right ventricular afterload, right heart failure, and premature death. Diverse genetic, pathological, or environmental triggers stimulate PAH pathogenesis culminating in vasoconstriction, cell proliferation, vascular remodeling, and thrombosis. Current concepts suggest that PAH pathogenesis involves three primary processes: vasoconstriction, cellular proliferation/vascular remodeling, and thrombosis.
The molecular mechanism underlying PAH pathophysiology is not known yet, but it is believed that the endothelial dysfunction results in a decrease in the synthesis of endothelium-derived vasodilators such as nitric oxide and prostacyclin. Moreover, stimulation of the synthesis of vasoconstrictors such as thromboxane and vascular endothelial growth factor (VEGF) results in a severe vasoconstriction and smooth muscle and adventitial hypertrophy characteristic of patients with PAH.
Between 11% and 40% of patients with Idiopathic pulmonary arterial hypertension [IPAH] and 70% of patients with a family history of PAH carry a mutation in the gene encoding bone morphogenetic receptor-2 (BMPR2). However, penetrance is low, carriers have a 20% lifetime risk of developing pulmonary hypertension. Therefore, “multiple hits” are probably needed for the development of PAH. In pulmonary hypertension associated with left heart disease (PH-LHD), raised left atrial pressures result in secondary elevation of pulmonary pressure. In pulmonary hypertension owing to lung disease or hypoxia (PH-Lung), raised pulmonary arterial pressures result from mechanisms such as vascular destruction and hypoxic vasoconstriction. In chronic thromboembolic pulmonary hypertension [CTEPH], mechanical obstruction of the pulmonary vascular bed, is the primary process. Incidences are estimated to be 1-3.3 per million per year for IPAH and 1.75-3.7 per million per year for CTEPH; the prevalence of PAH is estimated at 15-52 per million. Pulmonary hypertension is more common in severe respiratory and cardiac disease, occurring in 18-50% of patients assessed for transplantation or lung volume reduction surgery, and in 7-83% of those with diastolic heart failure.
While there is currently no cure for PAH significant advances in the understanding of the pathophysiology of PAH have led to the development of several therapeutic targets. Besides conservative therapeutic strategies such as anticoagulation and diuretics, the current treatment paradigm for PAH targets the mediators of the three main biologic pathways that are critical for its pathogenesis and progression: (1) endothelin receptor antagonists inhibit the upregulated endothelin pathway by blocking the biologic activity of endothelin-1; (2) phosphodiesterase-5 inhibitors prevent breakdown and increase the endogenous availability of cyclic guanosine monophosphate, which signals the vasorelaxing effects of the down regulated mediator nitric oxide; and (3) prostacyclin derivatives provide an exogenous supply of the deficient mediator prostacyclin.
There are various drugs approved for the treatment of PAH: inotropic agents such as digoxin aids in the treatment by improving the heart's pumping ability. Nifedipine (Procardia) and Diltiazem (Cardizem) act as vasodilators and lowers pulmonary blood pressure and may improve the pumping ability of the right side of the heart
Bosentan (Tracleer), ambrisentan (Letairis), macitentan (Opsumit), etc. are dual endothelin receptor antagonist that help to block the action of endothelin, a substance that causes narrowing of lung blood vessels. There are others which dilate the pulmonary arteries and prevent blood clot formation. Examples of such drugs are Epoprostenol (Veletri, Flolan), treprostinil sodium (Remodulin, Tyvaso), iloprost (Ventavis); PDE 5 inhibitors such as Sildenafil (Revatio), tadalafil (Adcirca), relax pulmonary smooth muscle cells, which leads to dilation of the pulmonary arteries.
Sildenafil is shown to be efficacious in therapy for humans with pulmonary arterial hypertension (Anna R Hemmes et al J. Expert Review of Cardiovascular Therapy, 4(3), 293-300, 2006)
U.S. Pat. No. 5,570,683 discloses method for treating or preventing reversible pulmonary vasoconstriction in a mammal such as PAH using combination of inhaled nitric oxide and therapeutically-effective amount of a phosphodiesterase inhibitor; wherein said phosphodiesterase inhibitor is administered before, during, or immediately after nitric oxide administration.
U.S. Pat. No. 7,893,050 discloses therapeutic combination, comprising an effective amount of fasudil and sildenafil, for treating pulmonary arterial hypertension.
European Patent No. EP 1097711B1 discloses use of Sildenafil in the manufacture of a medicament for treating or preventing pulmonary hypertension.
U.S. Pat. No. 8,324,247 discloses method for treating pulmonary arterial hypertension (PAH) by blocking both 5-HT2A and 5-HT2B receptors in a pulmonary artery such as N-Methyl-L-prolinol.
U.S. Pat. Nos. 9,474,752 and 8,377,933 discloses method for treating a pulmonary hypertension condition in a human patient, using combination of ambrisentan and agent selected from the group consisting of sildenafil, tadalafil and vardenafil.
Histamine stimulates only H1- and H2-receptors, since combined H1- and H2-receptor antagonism prevented almost all of the cardiovascular actions of histamine. (Tucker a. et al American J of Physiology, 229, 1008-1013, October 1975).
In addition to these established current therapeutic options, a large number of potential therapeutic targets are being investigated. These novel therapeutic targets include soluble guanylyl cyclase, phosphodiesterases, tetrahydrobiopterin, 5-hydroxytryptamine (serotonin) receptor 2B, vasoactive intestinal peptide, receptor tyrosine kinases, adrenomedullin, rho kinase, elastases, endogenous steroids, endothelial progenitor cells, immune cells, bone morphogenetic protein and its receptors, potassium channels, metabolic pathways, and nuclear factor of activated T cells.
Despite a certain success achieved in recent years, many patients with PAH are not adequately managed with existing therapies.
Thus, there is a need to provide a method of treating hypertension with the help of a drug which gives adequate therapeutic effect with minimal side effects and maximum therapeutic effect.